How much biogas production potential is there in Iowa?

 Are you interested in the potential anaerobic digestion holds in the state of Iowa? A new, easy to use tool, the Iowa Biomass Asset Mapping Tool (IBAM), may be just what you are looking for (available at www.ecoengineers.us/ibam). IBAM is an economic analysis tool integrated with geographical information systems and was produced as a collaborative product of Dr. Mark Wright of Iowa State University and the Des Moines based company EcoEngineers. 

The model is an online, GIS, interactive map where a user can click on different layers of data to study  the biogas resource potential available or conduct an initial screening of where a potential project could be sited based on both feedstock and infrastructure availability. The IBAM tool provides estimates on production of a wide variety of biogas-based feedstock data including crop residues, manures, and industrial co-products.

The users can assess the potential availability of animal manure, crop residue and determine what co-location opportunities with existing biodiesel, ethanol, food and paper manufacturers exist. In addition to the feedstock data, there is also energy infrastructure data on the locations of natural gas pipelines, electric and gas service territories, and existing power plant locations. This helps users identify potential locations for optimizing substrates available for gas production and for best using the produced biogas.

A complementary tool to the GIS map is a preliminary economic assessment spreadsheet. The downloadable spreadsheet provides users the availability to modify inputs and assumptions to conduct a preliminary economic evaluation for a potential biogas project. Both the IBAM map and spreadsheet will require more robust analysis and engineering designs for any project moving forward, but these publically available tools can help users from the private and government sectors to conduct an initial project screen or quantify the potential for biogas projects in Iowa.

So that’s all great if you are a biogas expert, but really, what is anaerobic digestion?

Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable material in an environment with no oxygen. As these microbes break down the organic matter they make a mixture carbon dioxide and methane with trace amounts of hydrogen sulfide, ammonia, water vapor, and other gasses that we call biogas. This occurs in a series of three steps: in the first large particles are brown down into things like carbohydrates. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria turn those organic acids into acetate, which methanogens convert into methane and carbon dioxide. This is a naturally occurring process, its just when we do it in a digester we are trying to make it happen faster and control where the produced methane goes. So this organic material goes in and what comes out is the methane, which can be combusted to make heat or electricity and stabilized organic matter which still has the fertilizer value (the N, P, and K) it had when it went in.

This almost seems too good to be true right, we still get all our fertilizer value but we can get energy as too! Well, in some cases it can be a great deal, but anaerobic digesters can be expensive to build and sometimes challenging to manage, so they aren’t the right fit everywhere as often times we need an economy of scale to make them cost affordable. Thought not the final solution, hopefully more tools like the IBAM model will help us figure out where anaerobic digestion has the potential to be successful. 

Bigger Pigs, More Manure – A Follow-up

On April 23^rd I posted a blog entry called “Bigger Pigs: More Manure and Impact on Facility Design.” Though they didn’t all get shared on the post (note there are like zero comments there), but I did get a few helpful emails and had several good conversations about it with different colleges.

As I discussed there were a few (lots) assumptions that went into making that estimate, but a big one was the growth curve of the pig. I pulled a growth curve out of the literature and rolled with it, thinking it would probably be ok; I mean I needed something to work with, but after looking it over, that growth curve seemed low - I mean it wasn't bad, the average daily gain was 1.8 lb/day, but as several people pointed out not only are we growing bigger pigs, but finish times are often decreasing as well. As I got to thinking about it, I wondered what does this mean for manure production?

So to answer this question I needed some growth curve data to work with. As a starting point I found a table published in National Hog Farmer showing that in 1980 a typical finish pig (at least at the research site they found a useful study from) had an average daily gain of about 1.50 lb/day, took 105 days to get to market, and had in and out weights of 49 and 206 pounds respectively. The same report said that in 2001, a typical rate of daily gain was 1.95 lb/day, the pig took 102 days to get to market, and had in and out weights of 62 and 260 respectively. As a point of references, the latest PIC wean-to-finish manual suggested the expected performance of a grow-finish pig was 2.03 lb/day going from 60 to 270 lbs (103 days), with optimized performance at 2.36 lb/day going from 60 to 270 lbs (89 days)

So where does that leave us? Well, it seemed like the best place to start was with the model I developed in the last post, but modifying it with different growth curves and seeing what happened. In this work I’ll focus on just grow-finish average manure production and look at the four new cases, i.e., 1.5 lb per day, 1.95 lb per day, 2.03 lb/day, and 2.36 lb/day and going to finish weights of 206, 260, 270, and 270 lbs and using initial weights of 49, 62, 60, and 60 respectively. Just for fun I'll compare that to the manure production I estimated from the growth curve with 1.8 lb/day with a start and finish weight of 60 and 270 lbs.

In the first case (that 1980 pig) I got an average manure production of 0.96 gallon per day, or about 100 gallons of manure being produced as I get that pig to market. In the second case (that 2001 pig) I got 1.15 gallons per day, or 117 gallons of manure produced in the time it took to get the pig to market. Finally, for the average 2015 pig I got 1.17 gallons per day, or 120 gallons of manure to get that pig to market. Finally, what about that optimized 2015 pig? I’d estimate about 1.17 gallons per day from that pig, but only 104 gallons to get it to market because of the more rapid finish time.

So, bringing it all back - what does this mean to us. To me, it still looks like bigger finish weights will lead to more manure, but improved genetics that are leading to larger daily gains and faster finish times. As a result, even though we are getting more manure on a daily basis, our total volume of manure per amount of pork produced seems to be going down.

Nutrient Management Spotlight - The Late Spring Nitrate Test

Don't feel neglected, the latest Manure Scoop has arrived. Hope everyone is having a successful planting season; may the leaves be green and your soils fertile (from that wonderful manure).

As you might have saw the new IMMAG newsletter just came out (you can find the complete newsletter at http://www.agronext.iastate.edu/immag/updates/may2015update.html). So I'm going to borrow one of the articles I wrote for the newsletter and post it here too. This one is on using the late spring nitrate test.

This year as part of the Manure Applicator Certification program, we asked you what you were doing as part of the Nutrient Reduction Strategy to reduce nutrient loses from your farm. One of the more popular answers was switching to split fertilizer application. Some potential benefits of split fertilizer application may include reduced opportunity for nitrate loss through leaching and denitrification, the potential to use less fertilizer, having less investment in the field if you are forced to replant to soybean after weather related losses or planting delays, or even slightly delaying to get additional information about this year’s markets and growing conditions. However, there can be concerns about the costs of making a second fertilizer application trip across the field, or even if the weather will permit this fertilizer application.

Often times when we switch to split application our general plan is be to apply 50-60% of the nitrogen recommendation in the fall or early spring, and then to sidedress the remaining 40-50% into the growing crop. An alternative approach is to determine our sidedress amount using the late spring nitrate test (LSNT). The late spring nitrate test is a nitrate only soil test where soil samples are taken to a depth of 12 inches when the corn plant is 6-12 inches tall. This test is supposed to inform us about available nitrogen concentrations in the soil just as our corn growth, and nitrogen need, is about to take off. In using the results you’ll want to break your field up into different management zone, parts of the field that have similar management histories and soil types (a management zone probably shouldn’t be any bigger than 10 acres). Within each management zone 16 to 24 soil cores should be collected. As these samples are collected you need to make sure that any banded fertilizer or manure isn’t biasing your results; sampling in a pattern relative to the corn (or banded fertilizer) row can help eliminate the effect of the banded application. For example, go to the first sampling location in your management zone and pull the first soil sample in the row, then move to your next sampling spot and pull the soil sample one-eighth the distance between rows, go to your next sampling location and pull the sample one-fourth the distance between rows, and continue this pattern.

Although this may seem a little complicated, the real difficulty starts in interpreting the results. Iowa State research says corn needs 25 ppm of nitrate-nitrogen in the top 12 inches of soil to produce maximum yield; however, the interpretation of the results vary with cropping system, manure history, and even weather conditions prior to and after sampling. Selecting the “critical” soil nitrate concentration (the one you are trying to achieve) is one of the more difficult parts of using the late-spring nitrate test to make management decisions.

Table 1. Nitrogen fertilizer recommendations for corn on manures soils.

	Recommended N rate
Soil Test	Excess rainfall	Normal rainfall
ppm NO3-N	lb N/acre	lb N/acre
0-10	90	90
11-15	0	60
16-20	0	0-30
> 20	0	0

In fields that have received manure, a “critical” soil nitrate-nitrogen concentration of 15-20 ppm nitrate-nitrogen is recommended. You’ll note that this is lower than non-manured fields; this is because the manure application provided more organic nitrogen that will be mineralized throughout the growing season and become plant available, but isn’t detected by this test. Based on your sample results you can then calculate the amount of nitrogen that would be recommended to sidedress. The formula for calculating nitrogen application is if your soil test was greater than 20 ppm then 0, otherwise (20 ppm - soil test nitrate) * 8 = lbs of N/acre to apply. Alternatively, table 1 provides a way to select a sidedress nitrogen application rate. In this table excess rainfall would be May precipitation that exceeded 5 inches, normal rainfall should be used for other cases.

As with any new fertility management program, first-time users are encouraged to experiment with the test in small areas before using it to guide fertilization on all their fields. As with most recommendations this test is intended to maximize profits when used across many years and sites, not to give the “perfect” rate in a specific year.

For more information related to using the Late Spring Nitrate Test please see, ISU PM1714-Nitrogen Fertilizer Recommendations for Corn in Iowa. More information on best management practices for reducing nutrient loss from agriculture can be found in SP435 - Reducing Nutrient Loss: Science Shows What Works.

Manure chemistry: What’s happening to my phosphorus

What happens to manure from the time it is excreted until it is land applied? Well, that’s simple right, we put it in some sort of storage (a pit, a lagoon, a Slurrystore, a concrete or earthen basin, or store it dry on a stacking pad) and it just stays there, waiting for us to land apply it and take advantage its fertility. Of course, it is not really that simple; manure waits for no man (or woman), while it is sitting there waiting it is changing (it is alive after all – it’s filled with microbes). Because of this, almost no matter how we store the manure, these microbes will be doing something – breaking down some of the organic material in the manure, converting elements from one form to another, or just causing things to be different.

This process can be complex and still isn’t completely understood, but we can give some generalities about what is happening and understand at least some of the jargon that gets thrown around. There are many ways to cover this topic, but what I am going to try to do is break it down by element, carbon, nitrogen, phosphorus, ect. In this post, I’m going to turn my attention to phosphorus.

Phosphorus

When we think of phosphorus we are probably most familiar with it in some of the fertilizer compounds we can purchase, such as mono-ammonium phosphate, di-ammonium phosphate, or triple super-phosphate. Otherwise we might think of it as the plant-available ion orthophosphate. Regardless of the actual chemical form of phosphorus we purchase, the analyses of phosphorus fertilizer is usually given as phosphate (P2O5).

       

Alright, so back to phosphorus in manure. As we’ve discussed previously, about 70% of the phosphorus consumed by the animal will be excreted, so it ends up in the manure. The actual amount is dependent on lots of things, animal age, species, diet composition, and numerous others, but the majority of the P we feed, will be in the manure. At the time of excretion, virtually all the phosphorus is associated with the fecal, or solid, material ( > 90%), but as the manure ages and the organic material in it breaks down from microbial action, more and more of the phosphorus becomes dissolved in the liquid fraction. For example in swine manure from a deep-pit, a good guess is that around 75% of the phosphorus will be dissolved reactive phosphorus (ortho-phosphate).

Although numerous fractionations schemes can be used to fractionate the manure phosphorus into pools of different characteristics and availabilities, the use of this information remains academic. From a practical standpoint, there are really four basic types of phosphorus in manure: dissolved inorganic phosphorus, precipitated inorganic phosphorus, and dissolved organic phosphorus, and particulate organic phosphorus.

Essentially all the dissolved, inorganic P in manure exists in the form of orthophosphate (or other derivatives of phosphoric acid (H₃PO₄, H₂PO₄^-, HPO₄^2-, and PO₄^3-). The exact form of this phosphate is dependent on the pH of the manure, but most of it will be either H₂PO₄^- or HPO₄^2- under normal pH conditions. This phosphorus is essentially equivalent to what would be purchased if you were to buy a mineral phosphorus fertilizer. As we store manure, two things can happen to this phosphorus, it can undergo biological immobilization and be incorporated into an organic molecule or it can react with something in the manure (often iron or calcium) and become a solid material (it is precipitated).

When we focus on the organic phosphorus, there are again two types, particulate phosphorus and dissolved organic phosphorus. In general, the flow is from organic phosphorus towards dissolved reactive phosphorus, this happens through a process called mineralization. Mineralization is just a way of saying broken down from an organic form (associated with carbon) to an inorganic from (usually H₂PO₄^- or HPO₄^2-). This process is mediated by micro-organisms living in the manure, as these “bugs” eat carbon to provide themselves with energy they break off the phosphorus and either use it to grow their cells, or release it into as dissolved phosphorus into the manure.

The final type of phosphorus is precipitated phosphorus. This is inorganic phosphorus that has reacted with something else in the manure and formed a particle. In general, I tend to think of this of being of less importance in manure, but in some cases we might add iron to our manure (as a way to acidify it – think for instance alum or ferric sulfate as amendments in poultry litter) and this makes it so our manure can hold less phosphorus in dissolved forms.

So now that we have talked about forms, where does this leave us? What we have seen is that phosphorus can change forms, but all these forms stayed in the liquid. This means that if the phosphorus was there to start with, it will be there when we land apply it (unless we spill some or if rainwater leaches through it and then flows away). However, just because it is there, doesn’t mean we’ll be able to capture it an land apply it. Since the particulate phosphorus is attached to solids, it is distributed like the solids. This means that the sludge rich manure often will have higher phosphorus contents and watery manure from the surface will have less. To get uniform phosphorus concentration we want to either get those solids evenly mixed in the manure, or to get as much of the phosphorus as possible into a dissolved form.

Bigger Pigs: More Manure and Impact on Facility Design

I was sent a question today that asked “why are more swine farmers hauling in spring and fall, why is this occurring, and what is a good number to use for sizing your manure storage?” So a nice multi-part question.

This is a great question for many reasons. At its surface, it seems easy, but I enjoyed it because it made me critically think about data I look at all the time. In particular, he had a follow-up question that said, how is this impacted by bigger finish weights (like 285 to 315 lbs.) This made me think of a blog article I read from Mike Brumm on the “Impact of heavier Sale Weights on Facilities.” In this post, he talked about space requirements for bigger pigs (no surprise bigger pigs grow better with more space) but didn’t touch on the manure topic, so what does bigger pigs mean for manure design (note – I’ll be thinking about mostly deep-pits, but the same ideas work for other storages).

So let’s start with some facts.
1. A recent survey of swine producers around the Midwestern US (lots of Iowa farms) conducted by the University of Minnesota suggested that about 50% of our farmers were applying once per year and 50% were hauling twice per year.
2. Increasing fiber in the diet (an example being DDGS) increases fecal output from the pig. An inclusion rate of 30-40% in DDGS increase fecal output by 5-10% .
3. Switching from nipple or cup waters reduces water consumption and potentially excess water reaching the manure storage, probably by roughly 5-10%.
4. Pig finish weights have tended to be increasing with 280 to 300 lb finish weights becoming more common.

So, that’s about it for cold, hard facts… what’s it mean and what else do we have to work with. So we know more farmers are pumping more often; is this because they have to because of storage constraints, a desires to apply more manure in the spring closer to planting time, issues with fall weather and getting the manure applied, or some combination of all three. From the survey we can’t tell, but I tend to think it all these factors coming together.

We also have competing factors for DDGS inclusion in the diet and improvements in watering systems to reduce consumption, I’m going to call them a wash, but at some farms they certainly could be making a difference. Finally, we know that pigs are getting and it seems intuitive that bigger pigs will make more manure per day.

So how do we typically estimate manure production from pigs, i.e., what’s your number? Well, in these my old go-to references on this have been the ASABE Manure Production and Characteristics standard, Midwest Plan Services (I use the Livestock Waste Facilities Handbook), or an extension publication from your local land grant university, but do these numbers still apply with bigger pigs?

Some colleagues and I recently had a project were we collected manure samples from swine manure deep-pits at right around 60 farms for a little over a year. As part of this study every month we measured the depth of manure in the storage pit, which allowed us to calculate manure accumulation (or production since we were catching all the manure) rates. There were a variety of diets being fed, the pigs were at various growth stages, some barns were wean-to-finish, some were grow-to-finish, and they had some different types of watering systems… in short, they were real production barns.

We found that on average manure was accumulating at a rate of 1.3 gallons of manure per pig per day (including any washwater used on the site). This was only a little above the ASABE manure production standard rate which suggests about 1.2 gallons per pig per day, so it seems reasonable. However, we saw lots of variation between barns, though they average 1.3 gallons per pig per day, the standard deviation was 0.4 gallons per pig per day. This means there is a lot of variation between barns, and in particular barns have a 25% chance of making more than 1.5 gallons per pig per day.

Now for the fun part, how will bigger pigs impact manure production? You’d think there would be lots of data floating around about how manure production changes with pig size, but it turns out that’s not typically how we collect the data. Its normally collected as amount of manure produced over a finishing cycle. So to get an estimate of manure production I used a couple of techniques. The first was looking back at some old manure production standards we use to provide an estimate of manure production per 1000 lbs of animal mass; using this information we can estimate the manure production rate for lots of different pig sizes. For the second approach, I used a phone-a-friend and to Brian Kerr, an animal scientist from USDA ARS who is my go to contact for swine diet stuff. He used some feeding trial data and the MWPS data and prorated it so that manure production didn’t increase quite as quickly in larger pigs as he thought the constant method over-predicted excretion from lager pigs. So these two methods are shown in the figure below. We expect actual manure production to fall somewhere between those two lines.

Figure 1. Swine manure production as a function of body weight.

Alright, now the fun part, to actually use this we need a swine growth model. I picked one I thought looked about right (shown below) and started with some math. Remember, what we wanted to know was how larger finish weights were impacting our manure number. What I’m showing in figure 3, is the number I’d pig for different finish weights. What this data suggests is that be switching from 250 lb. finish weights to 300 lb. finish weights would increase average manure production during the finish cycle by 13-17%, a pretty sizable increase. However, the actually increase in manure production is larger than this, by raising pigs to 300 pounds instead of 250 pounds would increase manure production by around 16 to 21%.

Figure 2. Body weight as a function of days on feed.

Figure 3. Average manure production rate based on finish weight.

 So where does this leave us? So for a grow-finish operation finishing at 285 lbs my average manure production number is right around 1.1 gallons per head per day, at 300 lbs it is right around 1.15 gallons per head per day. At this time I still use 1.2 gallons per head per day. For a wean finish operation I'd bring that number down to about 1.0 gallons per head per day. The truth is estimating actual manure amounts gets a little more complicated - do you double stock during the wean phase? When we start selling from barn how quickly is it empty? Is there down time between turns? Beyond that it seems like bigger pigs should lead to a bit more manure, but water and feeder systems and barn washing techniques probably still have as big of impact on manure amount variability that it is hard to see the impact of bigger pigs.

Thinking about animal production and sustainability

Last week I had the opportunity to give some freshman agricultural engineering students a tour of a beef feedlot near Ames. I enjoy doing this as I often get lots of interesting questions and discussion about animal production. There is great variety in the student’s agricultural backgrounds. Some grew up on animal farms while others have never seen an animal production facility before; this adds a degree of difficulty to discussing the topic, as you have to keep both audiences interested and engaged.

This means it’s important to cover the basics, i.e., what are we trying to accomplish at these farms, what are potential issues we might have, and what are the basic principles behind what we are doing. We are providing the animals with food, water, and protection to keep them healthy, happy, and growing quickly to make a product… meat, eggs, milk, or fiber. This makes sense as a first cut approach to thinking about the farm and discussing about design for animal needs, but to me the real excitement is a discussion on nutrient cycling at animal farms, specifically talking about the manure.

Nutrients are constantly cycling on farms and how we manage these cycles controls how effective we are at achieving our production goals while limiting environmental impact (of course mother nature plays a pretty big role in the nutrient cycling as well). At animal production facilities our goal is to take nutrients (in the form of animal feed ingredients) and change them into a more valuable form, specifically we are trying to convert them into tasty proteins. What we really want is to get as many of those nutrients as possible incorporated into our animals and then to sell them, unfortunately, only about 10-30% of the nitrogen and phosphorus we feed to animals ends up in the end products (meat, milk, or eggs). The remaining portion of these nutrients is excreted in manure.

Because of these relatively low protein conversion efficiencies (10-30%) many scientific arguments suggest that we need to eat less meat to promote sustainability. However, what many of these arguments fail to consider is that recycling of animal manure in integrated farms (farms with both crop and animal production) keeps a substantial share of the N, P, and K excreted by the animals circulating within agro-ecosystems, and in doings, means those proteins can still be used. This means our actual protein conversion efficiency can be substantially higher than those calculated just by calculating what percent of the nitrogen fed to an animal ends up in its body.

For example if I was to make a guess for swine farms in Iowa, I’d say about 20% of the N we feed ends up in the pig, 15% ends up lost as ammonia volatilization during manure storage, and the remaining 65% gets land applied to serve as a crop fertilizer for next year’s crop. This means that we are really looking at a protein conversion efficiency a lot closer to 85% that the 10-30% we might originally suggest. Of course, it gets a little more complicated as the crop will use not all of the nitrogen we apply to our field; some will be lost during the growing season. As a first guess I’d say about 20% of the applied N will be lost (though it depends on the production practices, crop rotation, soil and weather conditions during the growing season, as well as numerous other factors), but even including this in the calculation we still would estimate a conversion efficiency of around 75%.

Getting our students to start think about these issues, understanding the challanges facing production agricultural, and giving them an opportunity to get their hands a little dirty are crucial steps in continuing to move agriculture forward and getting the next generation of Agvocates ready to raise to the challenges ahead.

How do you determine N, P, & K needs for your farm?

 This may seem like a very basic question, and you are probably thinking I've been doing this for years, I think I have this part of the equation figured out (or perhaps the more you've learned the less certain you are on this part). In either case, I encourage you to keep reading, because while I agree this topic is fundamental it is anything but basic.

When I think about this topic there are two concepts that immediately spring (season joke for those of you counting at home) to mind: mass balance & dose-response. You might have different names for them, but since I’m trained as an engineer I've had them ingrained in me. However, the important point isn't what we call them, but what they tell us and what they mean.

So what is the “mass balance” approach? In the mass balance approach we often use and equation like:

In – Out = Change in Storage

There are a few ways to conceptualize this, but to me the easiest analogy was always the haymow (if I grew up on a swine farm it might have been a gain bin, or if I was a small child growing up on the farm today perhaps I’d think of a commodity shed). For those of you not versed in hay making the haymow refers to that place where you store hay, particularly the upper part of the barn (alternatively called hay loft). So how does this analogy work? Well if “ins” are greater than “outs” we’d be accumulating more hay in the barn. This would happen every summer, we’d make hay and stack it in the barn. We’d still feed some hay every night, but thankfully we were making it at a faster rate than we were using it. Come winter time the opposite would occur, we’d be making zero hay (since the world was frozen and snow covered), but we’d still be feeding some to the cows every day. Clearly the goal was to build up a big enough surplus in the summer so that our supply would last us until the next in-flow event (harvest).

So, what’s the analogy to this in nutrient management? I’m going to talk about it from the perspective of phosphorus, but it would work for any other nutrient as well. So in our hay example I used the “mow” as the place where we might have a change in storage, in the our agricultural system this will be our field, specifically from just above the soil surface to the depth our crop roots (I tend to picture about 4 feet deep, but it depends on the crop – Christmas trees root vastly differently than corn, just like alfalfa might root drastically different than soybean). We also have ins and outs, i.e., phosphorus is added to and removed from our field. Additions of phosphorus could include adding manure an putting on commercial phosphorus fertilizers (MAP, DAP, triple phosphate). Outs are things like the removal of phosphorus in the grain and crop residues we harvest, the loss of soil particles and attached phosphorus with erosion, or any dissolved phosphorus that flows from the field in surface runoff (this could also include phosphorus that moves vertically below the root zone of our crop as water leaches through the soil, but usually this is a really small amount in Iowa. A common management strategy would then be to try to balance the inflows and outflows of phosphorus from our field so that our soil stays like it was. That is we keep our inputs and outputs in balance so that we are neither gaining or losing phosphorus and hopefully our soil productivity remains the same.

An alternative way to view this is through the concept of dose-response. Dose-response refers to the relationship between the amount of a substance that an organism receives (the dose) and the effects on the organism (the response). In this approach the idea is to figure out how much response you might get from applying some dose. To get an idea of this one let’s think about the “do your chores” analogy. A parent might offer some economic incentive (an allowance) to try to achieve a desired response (for example unloading clean dishes from the dishwasher). In this case the parent would try to pick the lowest amount that would motivate their child to perform the task. The parent could of course pay more than this amount, but they don't receive any additional benefit from it.

In an agricultural system this would be akin to developing a curve that would predict the relative increase in yield that would be achieved by adding a certain amount of some nutrient. Again it will work on a variety of nutrients, but the one I’m going to pick as an example is nitrogen (because the work has all been done for me – see the maximum return to nitrogen tool @ http://extension.agron.iastate.edu/soilfertility/nrate.aspx). I’ve given an example of what this curve looks like for Iowa (at least on average) for corn following soybean. Just like the parenting analogy, our goal here would be the same – to provide just enough of what we are dosing (nitrogen addition) to maximize our response (I’m showing yield in the graph below, but we are doing it to maximize profit from our acre of crop), so supplying just enough nitrogen so that the next bushel of corn the extra nitrogen we add to our yield, will just pay for the costs of putting that little bit more fertilizer on.

So where does that leave us? Well, I’ve introduced two concepts for you to think about here: The mass balance approach and the dose-response approach. In the next couple posts I'll try to give more detail about both for the cases of nitrogen, phosphorus, and maybe even potassium. I won’t promise any answers about which one is right for your farm, but hopefully it will get you thinking about it in a new way and I’ll give you some insight into my thoughts.

PS - If you are already itching for more information on this topic I actually introduced a comparison between the mass balance approach and the dose-response in the blog post Iowa manure management plans: Yield goal versus maximum return to nitrogen (which you can find here: http://themanurescoop.blogspot.com/2015/01/iowa-manure-management-plans-yield-goal.html.